In the first nationwide study to examine the link between autism and air pollutants commonly found near freeways, researchers have found that high exposure to these toxic contaminants during pregnancy raises the odds a child will develop autism. Exposure in the third trimester leads to the greatest risk, according to researchers, while exposure before conception or after birth did not increase autism risk.

Among those exposed before birth, male offspring were more likely to develop autism.

“We are confident saying that air pollution is a risk factor for autism,” said Michael Rosanoff, director of public health research at Autism Speaks, which helped fund the study. “But risk does not equal cause. Not all women who are exposed to air pollutants will have child with autism, and not all children with autism get it from air pollution.”

The researchers focused on whether and how much pregnant women were exposed to microscopic bits of solid matter and liquid, called “fine particulate pollution,” that are created by burning carbon-rich fuels. Common sources include gasoline- and diesel-fueled vehicles, power plants, industrial facilities, and wood fires.

The researchers in 2005 began to survey participants in the Nurses’ Health Study—a major study that has tracked the health of more than 100,000 women since 1989—on whether or not they had children diagnosed with autism.

In 2007, the team gathered more detailed information, such as where the women lived before, during, and after pregnancy. A control group was created by matching up months of conception and birth between children with autism to those without.

Researchers then pared down the groups—245 children with autism and 1,522 children who did not suffer from the condition—to mothers who did not move after pregnancy. This became important when the researchers began to statistically calculate the women’s and children’s exposure to fine particle pollution before, during, and after pregnancy, based on federal air-quality data from locations all over the country. The scientists also considered how close or far the women lived from freeways and other air pollution sources.

While scientists don’t yet know why some children develop autism and others don’t, Rosanoff says, there is more than enough information linking air pollution and autism to warrant better air-quality regulations.

“I think EPA’s efforts to reduce automobile emissions is a step in the right direction, but we need to know more about how pregnant women are exposed,” he said. “Then we can think of better ways to target reducing exposure with policy.”

Particle pollution is a well-established contributor to asthma and heart and lung diseases.

Lab studies have shown that the microscopic particles can be absorbed by the body and cause developmental and genetic damage, while studies on mice have proved that they affect the fetal nervous system and cause other metabolic changes associated with the development of autism.

“We’re all concerned about our environment,” Rosanoff added. “This is giving us additional information, but we need more research dollars, from the federal government in particular, to get actionable information.”

While autism diagnoses have surged from one in 150 children to one in 68 children in just eight years, research funding has lagged, according to Rosanoff.

“The funding per individual with autism has gone down over time,” he said, “and further, the research funding that goes to environmental factors is significantly less than the proportion that goes to other types of autism research.”

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